1. Field of the Invention
The present invention generally relates to bandpass filters, and more particularly to active bandpass filters.
2. Description of the Prior Art
In microwave applications it has proven economical to incorporate as many components (e.g. antenna, balun, filters etc.) as possible into a System-on-Chip (SoC) integrated circuit thereby to reduce dependence upon off-chip components. However, passive filters utilizing semiconductor passive components commonly suffers large insertion loss and large chip area. Accordingly, extremely careful engineering processes must be exercised in manufacturing if low chip yield is to be avoided. Such engineering efforts are expensive and are particularly undesirable where the critical circuit involves only a small portion of the SoC integrated circuit.
Alternately, negative impedance based on active components has widely been used to improve characteristics of microwave passive filters. One such method is realized by employing a active transistor in conjunction with a capacitive feedback coupled to a passive microstrip bandpass filter in hybrid technology (Chi-Yang Chang and Tatsuo Itoh, “Microwave Active Filters Based on Coupled Negative Resistance Method,” IEEE Trans. Microw. Theory Tech. vol. 38, no. 12, pp. 1879-1884, December 1990.). The same negative resistance circuit had also been applied to an active coplanar waveguide bandpass filter in GaAs technology (Masaharu Ito, Kenichi Maruhashi, Shuya Kishimoto, and Keiichi Ohata, “60-GHz-Band Coplanar MMIC Active Filters,” IEEE Trans. Microw. Theory Tech. vol. 52, no. 3, pp. 743-750, March 2004.). Specifically, the effective bandwidth of this negative resistance circuit is limited and has to be designed in accordance with the passband frequency. Another disadvantage is the volume sizes of these filters could not be effectively shirked due to the passive transmission line circuit.
Another type of negative resistance circuit utilizes a cross-coupled pair of active transistors operating in differential mode to accommodate broadband negative impedance. This method commonly requires the architecture of an active filter to be fully balanced and almost doubles the number of passive components compared to the conventional single-ended passive filter (Dandan Li and Yannis Tsividis, “Design techniques for Automatically tuned integrated gigahertz-range active LC filters,” IEEE J. Solid-State Circuits, vol. 37, no. 8, pp. 967-977, August 2002.). Furthermore, for volume-size considerations, monolithic active filters designed to operate at gigahertz range in silicon technology usually employs lump inductive components accompanying the skin loss, substrate loss, and mutual coupling (Shaorui Li, Nebojsa Stanic, Krishnamurthy Soumyanath, and Yannis Tsividis, “An Integrated 1.5 V 6 GHz Q-Enhanced LC CMOS Filter with Automatic Quality Factor Tuning Using Conductance Reference,” 2005 IEEE Int. Radio Frequency Integrated Circuits Symp. Dig., pp. 621-624, July 2005.) Thus, a further disadvantage is that an additional tuning system has to be implemented with this active filter to accommodate a constant center frequency and passband flatness among different chips. It is desirable, therefore, to provide a low manufacturing cost, low volume in size, and low complexity active bandpass filter for microwave applications (Ching-Kuang C. Tzuang, Hsien-ung Wu, Hsien-Shun Wu, and Johnsea Chen, “CMOS active bandpass filter using compacted synthetic Quasi-TEM lines at C Band,” IEEE Trans. Microw. Theory Tech. vol. 54, no. 12, pp. 4555-4548, December 2006.).